1. Field of the Invention
The invention relates to a folding machine for folding a stack of sheets, the folding machine comprising:                a frame;        means for pre-folding a stack of sheets along a crease line into a main plane such that a booklet with a straight back is obtained;        means for keeping the pre-folded booklet relative to the frame;        means for finishing the fold.        
2. Description of the Related Art
Such a folding machine is for example known from US 20100104399.
For finishing printed paper and press work, it is widely known to feed the sheets to a finishing station. Such a finishing station typically could stack the sheets into a stack of sheets, provide staples to attach the sheets to each other, trim edges and fold the stack of sheets into a booklet.
For folding a stack of sheets into a booklet, a number of techniques are known. An often used technique for folding sheets supplied by a printer is the knife folding technique. The sheets supplied by the printer are stacked and transported near two parallel rollers. A knife blade will then push in the center of the stack, such that the center of the stack of sheets is fed to the parallel rollers and a crease is arranged in the stack of sheets. The knife blade is moved up to, but not through the nip of the parallel rollers.
To sharpen the crease in the folded stack of sheets, the stack could be fed to secondary rollers, which are pressed together under a higher force, than the first rollers. This increased force will sharpen the crease.
When feeding the stack of sheets through the primary and secondary rollers, each sheet is provided with a crease. However, depending on the number of sheets and position of the sheets relative to the center of the booklet, the crease is more or less sharp. The sheets near the center of the booklet have typically more sharp creases than the outer sheets. This is the result of the pressure arranged on the outside of the booklet and the resilience of the sheets.
The sharpness of the creases also depends on the friction between the sheets. If the sheets are provided with a glossy finish the friction between the sheets is low, resulting in less sharp creases.
A further option is to provide a so called square back fold. For such a fold a roller is run along the back, while the sides of the back are held by for example to pressing bars. This is known from GB2401820. However, the known square back techniques are only possible for booklet having at least some sheets of paper. A booklet with only one sheet cannot be provided with a square back fold by these known techniques.
It is an object of the invention to improve the knife folding technique.